Fiber-reinforced polymer composites are well known and widely used. Polymers of improved strength and increased stiffness can be obtained by the use of an appropriate reinforcing fiber. Probably the most widely used reinforcing fibers are glass, carbon and aramid (or "Kevlar" which is a registered trademark of the E. I. du Pont de Nemours and & Co., Wilmington, Del.). Composites in which aramid is the reinforcing fiber are known for their light weight, high strength and stiffness, resistance to stretch, vibration damping and resistance to damage. Aramid-reinforced composites are found in wide use, particularly the aircraft and aerospace industries, and in a variety of sports equipment. Fibrous reinforcements can be in the form of either long or short fibers.
A difficulty encountered in the production of fiber-reinforced polymer composites is the high mixing energy required to mix or blend the base polymer with the reinforcing fiber. This is due largely to the fact that all the common reinforcing materials are solids. Furthermore, composites having long, continuous strands of reinforcing fibers cannot be formed easily in conventional mixers, so that special methods are required in order to manufacture such composites.
The base polymers used in making reinforced polymer composites such as those described above are generally conventional thermoplastics, such as polyethylene, polypropylene, ethylene-propylene copolymers, polyvinyl chloride (PVC), styrene and copolymers thereof, polyamides, polycarbonates, and polyesters such as polyethylene terephthalate. These polymers are thermoplastic and are either amorphous or semi-crystalline. They may be called flexible chain polymers, since individual monomers units in the polymer chain are free to rotate with respect to each other so that the polymer chain may assume a random shape.
Another class of polymers, quite different in properties from the amorphous and semi-crystalline thermoplastics are the crystalline polymers. These may be classified into three groups as follows: (1) polymers which are crystalline in the solid state but not in the liquid state, (2) polymers which are crystalline in solution in an appropriate solvent but which do not have a crystalline melt phase, and (3) polymers which have a crystalline melt phase.
The first group of crystalline polymers which are known as those which are anisotropic (i.e., crystalline) in the solid phase and which either decompose before melting or which have melting points but are isotropic in the liquid phase. Such materials are reported, for example, in the opening paragraph of U.S. Pat. No. 4,083,829.
The second group are liquid crystalline materials which are optically anisotropic in solution but which are not melt processable, i.e., they decompose without melting and they show no glass transition temperature. These are referred to as "lyotropic" materials and are often referred to as lyotropic "liquid crystal" polymers. Such materials are described, for example, in Hwang et al, J. Macromol. Sci.-Phys., B 22(2), 231-257 (1983) and in Polymer Engineering And Science, Mid October 1983, Vol. 23, No. 14, pages 789-791. Composites of such polymers with flexible chain polymers are also described in these references.
The third group of crystalline polymers are those which are anisotropic and highly oriented, even in the liquid phase. These materials have well defined melting temperatures, and are melt processable. These materials may be characterized as thermotropic and are often called thermotropic "liquid crystal" polymers. Such polymers have rod-like structure even in the melt state. Liquid crystal polymers of this type are described, for example, in U.S. Pat. Nos. 3,991,014; 4,067,852; 4,083,829; 4,130,545; 4,161,470; 4,318,842; and 4,468,364.
U.S. Pat. Nos. 3,991,014 and 4,318,842 cited supra also disclose that the compounds described therein can be used in belts of automobile tires and for plastic reinforcement without giving any details.
Shaped articles can be formed from melt processable liquid crystal polyesters as has been described in the art. Both fibers and shaped articles made of liquid crystal polyesters are reported to have high tensile strength. This is true in the case of shaped articles if the tensile strength measurement is made in the direction of fiber orientation. However, shaped articles made of liquid crystalline polyesters in the absence of reinforcement have been found to have an unacceptably low tensile strength measured in a direction transverse to the direction of fiber orientation. Thus, although the literature reports that shaped articles of liquid crystalline polyesters can be made by conventional methods, such as injection molding, such methods inevitably lead to the production of articles having poor strength in the transverse direction. Special shaping methods are therefore necessary. In addition, the high cost of such materials precludes their use for making shaped articles on more than a limited scale.
European Patent Application Publication No. 0030417, published June 17, 1981, describes methods for improving the processability of melt processable base polymers which comprises adding a minor amount (typically about 10 to 20 percent by weight) of a thermotropic polymer which is compatible with the base polymer. As described in the publication, a thermotropic polymer is a polymer capable of forming an anisotropic melt when heated to a particular temperature range. The anisotropic melt forming polymer (or "liquid crystal" polymer) must be compatible with the base polymer, which typically is a conventional polymer such as polyolefin, polystyrene, polyester (e.g. polyethylene terephthalate), polyphenylene oxide or copolymer thereof, or polycarbonate. No claim for achieving improved physical properties as compared to the base polymer is made, and the data show that the physical properties of the blends are no better than and in some cases are worse than those of the corresponding pure base polymers.
U.K. Published Patent Application No. 2,008,598 discloses a polymer composition comprising 20 percent or less, based on the total weight of polymer, of a rigid polymer, and a second polymeric material having flexible molecular chains. The rigid polymer is dispersed in the second polymeric material as microscopic particles of one micrometer or less.
U.S. Pat. No. 4,460,735 describes polymer blends comprising approximately 5 to 75 percent by weight, based on the total weight of the blend, of a polycarbonate and 25 to 95 percent of a melt processable wholly aromatic polyester. These blends or composites show somewhat improved mechanical properties over what one would expect from the rule of mixture. A tendency toward decreasing strength and modulus is seen at lower concentrations of the wholly aromatic polyester.
Only recently has the first commercial melt processable or thermotropic liquid crystal polymer (LCP) been introduced, even though such polymers have been described extensively in patent and technical literature. Such a polymer was introduced commercially by Dartco under the trademark "Xydar" according to Modern Plastics, December 1984, pages 14 and 16.
The properties of Dartco's "Xydar" are described in further detail in a paper presented at the 43rd Annual Technical Conference of the Society of Plastics Engineers, Inc., April 29-May 2, 1985, and published in Antec'85 Plastics, pages 769-772.
There is a need for new polymer composites which have properties comparable to those of presently known light weight, high strength polymer composites but which can be prepared at lower cost.